Recently, a communications system such as a mobile telephone system has come into widespread use. Such a communications system includes a transmission amplifier for amplifying a transmission signal. Since the transmission amplifier has a wide dynamic range of an output signal, a power supply voltage of the transmission amplifier is configured to be changed according to an output level of the transmission amplifier by using a voltage step-up/step-down switching regulator so as to effectively drive the transmission amplifier.
FIG. 1 is a block diagram illustrating a conventional voltage step-up/step-down switching regulator 10.
The block diagram of FIG. 1 illustrates a power supply circuit capable of controlling an output voltage Vout of the voltage step-up/step-down switching regulator 10 used as a power supply for a transmission amplifier between a voltage lower than an input voltage Vin of the voltage step-up/step-down switching regulator 10 and a voltage higher than the input voltage Vin by inputting a control voltage Vs varying according to the output level of the transmission amplifier.
The voltage step-up/step-down switching regulator 10 includes a reference voltage source 111, an error amplifier (EAMP) 11, a 100% duty cycle protection circuit 13, a step-down control circuit 14, a step-down driver circuit 15, a step-down switching transistor M1, a step-down rectification transistor M2, a step-up control circuit 17, a step-up driver circuit 18, a step-up switching transistor M3, a step-up rectification transistor M4, an inductor L1, a resistor R11, and a resistor R12. The control voltage Vs is input to the voltage step-up/step-down switching regulator 10.
The control voltage Vs is applied between one end (left end in FIG. 1) of the resistor R12 and ground. A feedback voltage of the output voltage Vout is applied to an inverted input terminal of the error amplifier 11 through the resistors R11 and R12, and a reference voltage Vref is applied to a non-inverted input terminal of the error amplifier 11. Output of the error amplifier (EAMP) 11 is input to the step-down control circuit 14 and the step-up control circuit 17.
An output terminal of the 100% duty cycle protection circuit 13 is connected to the step-up control circuit 17 so as not to produce a 100% duty cycle.
Further, a step-down triangular wave D_TRIWAV for performing PWM (Pulse Width Modulation) control in a step-down operation is input to the step-down control circuit 14, and a step-up triangular wave U_TRIWAV for performing PWM control in a step-up operation is input to the step-up control circuit 17.
Through the step-down driver circuit 15, the step-down control circuit 14 controls a gate voltage of the step-down switching transistor M1 comprised of a PMOS transistor and a gate voltage of the step-down rectification transistor M2 comprised of a NMOS transistor.
Through the step-up driver circuit 18, the step-up control circuit 17 controls a gate voltage of the step-up switching transistor M3 comprised of a NMOS transistor and a gate voltage of the step-up rectification transistor M4 comprised of a PMOS transistor.
An output voltage Vout17 of the step-up control circuit 17 is expressed by the following formula:Vout17=Vref(1+R11/R12)−Vs×R11/R12  (1)
According to the formula 1, when the control voltage Vs is zero, the output voltage Vout17 is equal to an output voltage of a normal voltage step-up/step-down switching regulator. However, when the control voltage Vs of a certain value is applied, the output voltage Vout17 drops according to an increase in the control voltage Vs.
There is a quick response power supply circuit having a wide control range formed by combining a step-up circuit converter and a quick response step-down element. However, in a method in which the conventional voltage step-up/step-down switching regulator 10 is used, when the control voltage Vs rapidly drops to rapidly increase the output voltage Vout from low to high, a voltage difference between the inverted input terminal and the non-inverted input terminal of the error amplifier 11 increases due to a delay in response, and the output of the error amplifier 11 rises, causing a step-up circuit to operate even when the output voltage Vout is lower than the input voltage Vin.
In the step-up circuit, when energy is stored in an inductor LX, current flows to ground, resulting in a large loss compared to a step-down circuit. Further, in the process of storing energy in the inductor LX, since power is not supplied to an output terminal of the voltage step-up/step-down switching regulator 10, response is slow compared to the step-down operation, causing a delay in rise of the output voltage Vout.
In the conventional method, responsiveness is improved by boosting a voltage and reducing the boosted voltage using a quick response step-down element such as a regulator. However, power supply efficiency is not improved.